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Harold McGee on Yogurt, Ice Cream and Tea

By The New York Times August 11, 2008 10:08 amAugust 11, 2008 10:08 am

Q: Since I’m a Greek-style yoghurt addict, which is expensive, I’ve begun to make my own, which is quite lovely. I make my batches in a crock pot, but I have read that some people leave innoculated milk on top of the frig or in the oven with just the pilot light on. Does this work? I’ve read that the innoculated milk must reach approximately 115 degrees in order for it to ferment into yoghurt, but if I can just leave it on top of the frig or in the oven without wasting energy that would be great.
— Posted by Amy

Harold McGee replies: I’ve been making yogurt from an heirloom culture twice a week for about a decade now, and I’ve found that you can do so at pretty much any warmish temperature, as long as you’re patient. The warmer the milk, the faster it sets, but you don’t really need a heat source. I scald the milk, let it cool to 115 degrees, wrap the container in a couple of layers of kitchen towels to slow the cooling down, and it’s usually set in 3-4 hours.

Q: Does your quickie ice cream chilling technique work for custard-based creams as well? How can I prevent iced tea from getting muddy as it chills?
— Posted by Willa Hutner

Harold McGee replies:Harold McGee replies: The quick ice cream method works even better for custard-style mixes, which come out very smooth with no stirring thanks to the egg yolk proteins and emulsifiers.

The way to get clear iced tea is to brew it slowly in the fridge, as Mimi Sheraton suggests in #66. Tea clouds because caffeine and astringent tannins are more soluble in hot water than cold: so when hot-brewed tea is chilled, some caffeine and tannins precipitate out of the solution in tiny solid particles. If you infuse the tea in the cold, then it never dissolves more caffeine and tannins than it can hold when cold.

Speaking of tea…

Q: What is the relationship between water temperature and caffeine extraction from tea leaves? Is it different for black and green tea?
— Posted by Marc

Harold McGee replies: It’s a straightforward relationship: the hotter the water, the faster and more completely the caffeine is extracted, no matter what kind of tea you’re brewing.

As for clear tea, you can also boil water, pour it over tea leaves, strain the tea into a bowl and let it cool on the counter. I do this and always have clear tea, cloudy only happens when I add a lot of ice while it is cooling.

For those of us who keep the house pretty cold, the easiest way to keep yogurt at a good incubating temperature is to use a picnic cooler with the bottle of warm yogurt and a second bottle filled with very hot water. Easy and controlled.

Another very reliable way to make small quantities of yogurt irrespective of your room temperature is to use a wide mouth thermos bottle. Just pour the inoculated milk in at 115°F and put the cap on. Let it sit on the counter (you can wrap it in a towel if you want to for additional insulation). Since the doubling time of the bacteria in the culture gets longer as the temperature goes down, (the temperature goes down approximately linearly with time), and the bacterial concentration is going up exponentially (though with a decreasing exponent), it all works out in your favor. Generally give it 12 hours, then take off the lid, cover the thermos with plastic wrap, and put it in the refrigerator. The culture continues to propagate even after it goes in the refrigerator though it gets vverrrry sluuuuggisshhhh as it cools down.